The invention relates to processing a spread spectrum signal.
In wireless systems, information typically is transmitted by modulating the information onto carrier waves having frequencies that lie within preassigned frequency bands. Radio frequency (RF) receivers demodulate the carrier waves to recover the transmitted information.
Spread spectrum communication systems spread transmitted signals over bandwidths much larger than those actually required to transmit the information. Spreading a signal over a wide spectrum has several advantages, including reducing the effects of narrow band noise on the signal and, in many situations, providing increased protection against interception by unwanted third parties. In a direct sequence spread spectrum (DSSS) system, the bandwidth of a transmitted signal is increased by modulating the signal onto a known pseudo-noise (PN) signal before modulating onto the carrier wave. The PN signal typically is a digital signal having an approximately equal number of high and low bits (or xe2x80x9cchipsxe2x80x9d), which maximizes the spectrum over which the signal is spread. A typical implementation of a DSSS receiver recovers the transmitted information by demodulating the carrier wave and then multiplying the resulting signal with a local replica of the PN signal to eliminate the PN signal. The DSSS technique offers heightened security because the receiver must know the PN sequence used in the transmission to recover the transmitted information efficiently. Other spread spectrum techniques include frequency hopped spread spectrum (FHSS).
In one aspect, the invention features a receiver for use in a spread spectrum communication system. The receiver includes an acquisition system configured to detect a transmitted spread spectrum signal by simultaneously correlating multiple search phases of a reference spreading signal against an output from a receiver channel; a demodulation system configured to recover data embedded in the spread spectrum signal by simultaneously correlating the spread spectrum signal with multiple possible data phases of the reference spreading signal over consecutive data periods; and a bank of correlation devices configured for use both in the acquisition system and in the demodulation system.
Embodiments of the invention may include one or more of the following features. Each of the correlation devices may be configured to compare the spread spectrum signal against a different one of the search phases when configured for use in the acquisition system and against a different one of the data phases when configured for use in the demodulation system. Each correlation device may include a multiplication element configured to multiply the spread spectrum signal with the reference spreading signal to produce a product output, and an accumulation element configured to produce an accumulation output by accumulating the product output over each of the data periods. Each correlation device also may include a delay element configured to phase shift the accumulation output by 180xc2x0. The accumulation element may be configured to subtract from the product output the 180xc2x0 phase shifted version of the accumulation output. The receiver also may include an analog-to-digital converter configured to sample the spread spectrum signal at a selected sampling rate, and the correlation devices may be configured to process the spread spectrum signal at a center frequency equal to approximately one-quarter the sampling rate.
In another aspect, the invention features a receiver for use in processing a spread spectrum signal containing data that is CCSK-modulated onto a pseudo-noise (PN) spreading sequence. The receiver includes an analog-to-digital converter configured to sample the spread spectrum signal at a selected sampling rate, multiple correlation devices, and a processing element. Each of the correlation devices includes the following components: a multiplication element configured to multiply the sampled spread spectrum signal against a copy of the PN sequence at a selected code phase and to produce a corresponding product output at a center frequency less than the sampling rate and greater than zero; and an accumulation element configured to accumulate the product output at the center frequency to produce an accumulation output. The processing element is configured to determine which, if any, of the accumulation outputs indicates alignment between the spread spectrum signal and the copy of the PN sequence in one of the correlation devices.
In yet another aspect, the invention features a method for use in receiving signals in a spread spectrum communication system. A transmitted spread spectrum signal is acquired by simultaneously correlating multiple search phases of a reference spreading signal against an output from a receiver channel. Data embedded in the spread spectrum signal then is recovered by simultaneously correlating the spread spectrum signal with multiple possible data phases of the reference spreading signal over consecutive data periods. A single bank of correlating devices is used both in acquiring the spread spectrum signal and in recovering the data embedded in the spread spectrum signal.
Advantages of the invention may include one or more of the following. An implementation efficient correlator structure may be used in a spread spectrum receiver system, which reduces the cost of the system and allows many correlators to be used to acquire and demodulate incoming spread spectrum signals. Increasing the number of correlators in the receiver system allows for more rapid and efficient signal acquisition. For example, many correlators may be used to search for a DSSS signal at search phases separated by less than one chip of the PN sequence used to spread the DSSS signal. The invention further reduces receiver cost by utilizing multiple correlators both for signal acquisition and for demodulation.
Other advantages of the invention will become apparent from the following description and from the claims.